Brake fluid is used to transmit the pressure exerted on a motor vehicle's brake pedal to the slave cylinders of the braking system. The most commonly used brake fluids in North America consist of glycol-based liquids categorized as DOT3 and DOT4 on the basis of the boiling point resulting from their particular composition. In order to prevent boiling of the fluid caused by overheating during use, DOT3 and DOT4 fluids are required to have a dry boiling point (with no moisture in fluid) of at least 401° F. (205° C.) and 446° F. (230° C.), respectively, so that proper brake operation is ensured under all temperature conditions.
All types of glycol-based brake fluid are hygroscopic. As a result of this property, they readily absorb moisture that reduces their boiling point.
Another important aspect of brake systems maintenance is the corrosive nature of some of their constituents, which, upon contamination of the brake fluid, progressively contribute to damage metallic tubing and other parts of the brake system. Most brake lines consist of steel lined with a copper alloy. Brake lines typically represent the largest surface area of metal in contact with brake fluid. A typical light-duty vehicle uses about 14 meters of such copper-lined steel tubing with an inside diameter of about 2.5 mm, for a total internal surface area of about 1,200 cm2. Therefore, corrosion of these lines contributes relatively large amounts of copper ions to the fluid. The master and slave cylinders and antilock brake system (ABS) include components made of steel, aluminum, zinc, or other materials that are also susceptible to corrosion as the fluid ages, its water content increases and its reserve alkalinity decreases. Dissolved iron is also known to appear in brake fluid after the initial amine corrosion inhibitor is significantly depleted and the dissolved copper level reaches about 200 ppm. By the time this event occurs, the brake fluid is highly corrosive and damaging to the brake system, and its replacement is called for regardless of its actual service time or mileage.
Corrosion inhibitors in new brake fluid inhibit corrosion initially. In conventional brake fluids, amines are included in the new brake fluid to inhibit corrosion and prevent damage to metal parts that operate in contact with the fluid. Corrosion inhibitors deplete with time, temperature, and environmental stress (for example, air, moisture, salts, ions, and other contaminants seeping into the fluid). As the corrosion inhibitors deplete, corrosion accelerates. As the brake fluid ages, its anticorrosive properties are measured in terms of reserve alkalinity; that is, the amount of amines remaining in the fluid to buffer the acidity resulting from breakdown of fluid constituents. Over time, thermal oxidation and volatization produce a significant reduction of the amine content and the concurrent decrease of anticorrosive properties. Tests have shown that the reserve alkalinity of DOT3 and DOT4 fluids is reduced to about 20 percent of its original value after 18 to 20 months of normal operation.
Corrosion in the brake system is harmful, and at some point internal corrosion may interfere with the proper operation of the brakes or antilock brake system (ABS). Copper deposition onto brake system components, particularly ABS seats and valves, is also undesirable and may interfere with the proper operation of the brakes or ABS. A finite level of copper ions in brake fluid with water will cause corrosion of ferrous metals like iron, cast iron, steel, and the like in the brake system. The level of copper ions in brake fluid with water (as low as 2% water) is an indicator of the virtual age of the brake fluid and the potential for corrosion of ferrous metals.
Copper is of interest in brake system corrosion because copper can form stable ions in water-free (dry) inhibited commercial brake fluid, and thus copper can begin corroding immediately upon contact with brake fluid in the presence of oxygen. Copper is also an oxidizer for other metals. Iron ions, on the other hand, have limited solubility in dry commercial brake fluid. Iron begins to corrode when brake fluid takes on water and/or the corrosion inhibitors become depleted. Iron may be oxidized by oxygen, water, and copper ions in the brake fluid. Copper oxidizes iron in the same manner as oxygen, with a copper ion gaining an electron from an iron atom, plating or depositing a copper atom on or near the iron substrate, and releasing an iron ion into solution. The copper level in brake fluid is directly proportional to the corrosivity of the brake fluid, or its potential corrosivity.
However, copper is not the only species of interest in brake system corrosion. Iron, zinc, tin, aluminum, and brass (an alloy of copper and zinc) are also found in brake systems, and these materials are susceptible to corrosion. Any of these materials, or their ions, may be referred to as a reactive constituent of brake fluid because of their susceptibility to corrosion.
Therefore, brake fluids also need to be checked and periodically replaced in order to prevent dangerous corrosion in the brake system. Accordingly, industry maintenance recommendations, where such exist, are typically based on service time and mileage of the vehicle.
U.S. Pat. No. 6,691,562 discloses an approach for estimating service time and/or mileage of brake fluid based on the recognition that the copper content in brake fluid is predictably related to time and mileage of vehicle operation. Thus, this correlation can be advantageously used to estimate milestones for maintenance purposes without regard to actual service time and/or mileage. Instead, copper content is adopted as a reliable indicator of a brake system's or a vehicle's “virtual age,” a term used to refer to the wear and tear on brake fluid resulting from actual mileage, actual service conditions, and/or time of service.